SHRIMP U–Pb zircon dating of the host rocks of the Cannington Ag–Pb–Zn deposit,southeastern Mt Isa Block,Australia

SHRIMP U–Pb zircon ages are reported from a paragneiss, a pegmatite, a metasomatised metasediment and an amphibolite taken from the upper amphibolite facies host sequence of the Cannington Ag–Pb–Zn deposit at the southeastern margin of the Proterozoic Mt Isa Block. Also reported are ages from a middle amphibolite‐facies metasediment from the Soldiers Cap Group approximately 90 km north of Cannington. The predominantly metasedimentary host rocks of the Cannington deposit were eroded from a terrane containing latest Archaean to earliest Palaeoproterozoic (ca 2600–2300 Ma) and Palaeoproterozoic (ca 1750–1700 Ma) zircon. The ca 1750–1700 Ma group of zircons are consistent with sedimentary provenance from rocks of Cover Sequence 2 age that are now exposed to the north and west of the Cannington deposit. The metasedimentary samples also include a group of zircon grains at ca 1675 Ma, which we interpret as the maximum depositional age of the sedimentary protolith. This is comparable to the maximum depositional age of the metasediment from the Maronan area (ca 1665 Ma) and to previously published data from the Soldiers Cap Group. Metamorphic zircon rims and new zircon grains grew at 1600–1580 Ma during upper amphibolite‐facies metamorphism in metasedimentary and mafic magmatic rocks. Zircon inheritance patterns suggest that sheet‐like pegmatitic intrusions were most likely derived from partial melting of the surrounding metasediments during this period of metamorphism. Some zircon grains from the amphibolite have a morphology consistent with partially recrystallised igneous grains and have apparent ages close to the metamorphic age, although it is not clear whether these represent metamorphic resetting or crystallisation of the magmatic protolith. Pb‐loss during syn‐ to post‐metamorphic metasomatism resulted in partial resetting of zircons from the metasomatised metasediment.     

THE PRESENT STATUS OF ZIRCON

Saxena (1966a) proposed that zircon could be authigenic and low-grade metamorphic, as well as magmatic in origin. He questioned the mechanical and chemical stability of zircon, the use of crystal morphology in correlation, and the use of roundness as a criterion of detrital origin. In so doing, he challenged the role of zircon in problems of stratigraphy, pedogenesis, and sedimentary, igneous, and metamorphic petrology. SAXENA'S(1966a, b) evidence for authigenic and low-grade metamorphic zircon is unconvincing, since conventional explanations adequately encompass his obser- vations, and do not conflict with established knowledge of zircon behaviour. During weathering, authigenesis, and metamorphism, zircon is shown to conform with the concepts that: (1) its removal and transport in solution is minimal, (2) zircon formation during authigenesis and low-grade metamorphism is insignificant. Rare occurrences of hydrozircon confirm this paucity of authigenic zirconium minerals. At high metamorphic grades zircon-transformation is favoured by the “wet” metasomatic processes of the amphibolite facies and impaired by the “dryness” of the granulite facies. The occurrence of zircon transformation in the granulite facies may also be prevented by incorporation of zirconium in the lattice of newly-formed pyroxene. Release of this zirconium during diaphthoresis of pyroxene-bearing granulites could give rise to new zircon. The formation of zircon during authigenesis, and its formation and transfor- mation during low-grade metamorphism, would diversify initially homogeneous assemblages; but Saxena has failed to prove that either process does in fact occur. It is contended that, since the positive correlation of zircon assemblages from co-eval sedimentary and metamorphic rocks of the Kingsbridge area is opposed to Saxena's views regarding the origin and behaviour of zircon, the roles of authigenesis in the formation of zircon, and of low-grade metamorphism in its formation and transformation, are insignificant or non-existent.     

Provenance variations in the Late Paleozoic accretionary complex of central Chile as indicated by detrital zircons

We present detrital zircon UPb SHRIMP age patterns for the central segment (34–42°S) of an extensive accretionary complex along coastal Chile together with ages for some relevant igneous rocks. The complex consists of a basally accreted high pressure/low temperature Western Series outboard of a frontally accreted Eastern Series that was overprinted by high temperature/low pressure metamorphism. Eleven new SHRIMP detrital zircon age patterns have been obtained for meta-turbidites from the central (34–42°S) segment of the accretionary complex, four from previously undated metamorphic complexes and associated intrusive rocks from the main Andean cordillera, and three from igneous rocks in Argentina that were considered as possible sediment source areas. There are no Mesozoic detrital zircons in the accretionary rocks. Early Paleozoic zircons are an essential component of the provenance, and Grenville-age zircons and isolated grains as old as 3 Ga occur in most rocks, although much less commonly in the Western Series of the southern sector. In the northernmost sector (34–38°30′S) Proterozoic zircon grains constitute more than 50% of the detrital spectra, in contrast with less than 10% in the southern sector (39–42°S). The youngest igneous detrital zircons in both the northern Western (307 Ma) and Eastern Series (345 Ma) are considered to closely date sedimentation of the protoliths. Both oxygen and LuHf isotopic analyses of a selection of Permian to Neoproterozoic detrital zircon grains indicate that the respective igneous source rocks had significant crustal contributions. The results suggest that Early Paleozoic orogenic belts (Pampean and Famatinian) containing material recycled from cratonic areas of South America supplied detritus to this part of the paleo-Pacific coast. In contrast, in the southern exposures of the Western Series studied here, Permian detrital zircons (253–295 Ma) dominate, indicating much younger deposition. The northern sector has scarce Early to Middle Devonian detrital zircons, prominent south of 39°S. The sedimentary protolith of the northern sector was probably deposited in a passive margin setting starved of Devonian (Achalian) detritus by a topographic barrier formed by the Precordillera, and possibly Chilenia, terranes. Devonian subduction-related metamorphic and plutonic rocks developed south of 39°S, beyond the possible southern limit of Chilenia, where sedimentation of accretionary rocks continued until Permian times.     

阿尔泰递增变质带中夹层石英岩的LA-ICP-MS碎屑锆石U-Pb年龄:对沉积时限及物源的限定

阿尔泰造山带广泛分布各种变质沉积岩并发育典型递增变质带,变质沉积岩变质之前的沉积时代与物源特征对于限定成岩历史以及造山带演化具有重要意义。文章对采自阿勒泰组变质带中石英岩夹层样品进行了岩相学分析并采用LA-ICP-MS方法对其碎屑锆石进行了U-Pb年代学分析。共获得100个谐和或近于谐和的碎屑锆石年龄,表面年龄分布范围为(443±5)Ma至(2682±19)Ma。碎屑锆石年龄主要集中在寒武纪(486~540 Ma)并具有527~535 Ma的年龄峰值,可能源于区域内同时代的岩浆活动。新元古代年龄约占1/4,少量锆石具有古中元古代甚至太古宙年龄。结合年轻碎屑锆石年龄以及直接侵入该变质带中的英云闪长岩年龄可确定石英岩原岩的沉积时限为早志留世—早泥盆世,其后发生变质作用。古老碎屑锆石在该地区缺乏对应的岩石,可能源于区内隐伏的古老基底岩石或邻区古老陆块。     

冀西北怀安地体高级变质表壳岩的锆石年代学研究

位于华北克拉通中部造山带中北段的怀安地体与内蒙孔兹岩带相接,出露高压麻粒岩和退变榴辉岩等多种高级变质岩,是洞悉华北克拉通古元古代构造演化历史的重要窗口。研究区变质表壳岩包括夕线石榴长英质片麻岩、石榴长英质粒状岩石以及紫苏黑云二长片麻岩。阴极发光图像特征显示研究区样品的锆石主要包括碎屑锆石和变质锆石,其中碎屑锆石具有岩浆结晶环带,而变质锆石为单颗粒或围绕着继承性碎屑锆石边部生长,内部结构均匀,Th/U比值较低。锆石LAICP-MS U-Pb定年结果与前人研究结果综合表明该区变质表壳岩石的碎屑锆石的207Pb/206Pb年龄主要集中在~2040Ma,其原岩形成时代与孔兹岩带变泥质岩石相近,均为~2.0Ga。变质锆石记录其变质时代为1957~1804Ma,结合前人对怀安地区变泥质岩和变基性岩变质作用和年代学研究结果,推测得出1.95~1.92Ga代表了峰期(高压)麻粒岩相变质时代,1.90~1.85Ga代表峰后减压阶段变质时代,而1.85~1.80Ga代表退变质晚期的时代。怀安地区变质岩石可能卷入了阴山陆块、鄂尔多斯陆块以及东部陆块间的先后碰撞造山过程,并持续较长时间(1.95~1.80Ga),最终拼贴为统一的整体。     

北秦岭西段冥古宙锆石(4.1～3.9Ga)年代学新进展

2007年王洪亮等报道在北秦岭西段火山岩中获得一粒年龄为4079±5Ma的冥古宙捕虏锆石。之后,对这一发现开展了深入的调查研究,我们除利用SHIMP技术方法对原4079Ma的锆石进行验证外,新获得了两粒~(207)Pb/~(206)Pb年龄为4007±29Ma和3908±45Ma捕获的变质成因锆石,表明早在4.0Ga已经有变质作用的发生,这或许说明在冥古宙时期地球已经具有相当规模和厚度的地壳。同时开展的岩石学研究表明,蕴含古老锆石的母岩属于火山碎屑熔岩类而不是火山熔岩。     

EVOLUTION OF ZIRCONS IN SEDIMENTARY AND METAMORPHIC ROCKS: A DISCUSSION

Saxena's (1966a) paper on the “Evolution of zircons in sedimentary and metamorphic rocks” is discussed. The arguments for the stability of zircon during sedimentary and metamorphic processes are reviewed. Saxena's idea of an authigenic origin of the majority of zircons in some sediments seems to be the result of a misidentification of authigenic titaniferous minerals as zircon.     

超高压变质岩的放射性同位素体系及年代学方法

深刻理解同位素在超高压变质及退变质过程中的地球化学行为对获得超高压变质岩准确并有明确意义的年龄值是非常重要的。对 Sm-Nd,Rb-Sr 同位素体系,只有变质矿物同位素体系达到平衡才能给出精确有意义的等时线年龄。研究表明,与副变质岩互层的细粒榴辉岩的高压变质矿物之间,或者强退变质岩石的退变质矿物之间,其 Nd,Sr 同位素可以达到平衡;然而高压变质矿物与退变质矿物之间 Nd,Sr 同位素不平衡。由于全岩样品总是含有数量不等的退变质矿物,因此石榴石 全岩 Sm-Nd 法或多硅白云母 全岩 Rh-Sr 法将有可能给出无地质意义的年龄。通常低温榴辉岩的高压变质矿物之间存在Nd 同位素不平衡。超高压变质岩多硅白云母所含过剩 Ar 主要源于榴辉岩原岩中角闪石在变质分解时释放出来的放射成因 Ar。因此,不含榴辉岩的花岗片麻岩多硅白云母基本不含过剩 Ar。对变质锆石成因的准确判断是正确理解锆石 U-Ph 年龄意义的关键。本文对不同成因锆石的判别标志及年龄意义做了总结,并指出将阴极发光图形,锆石痕量元素组成及矿物包裹体鉴定相结合是进行锆石成因鉴定的有效方法。高压变质或退变质增生锆石组成单一,是理想变质定年对象。然而变质重结晶锆石域常是重结晶锆石和继承晶质锆石的混合区,因而给出混合年龄。只有完全变质重结晶锆石才能给出准确变质时代。     

Synchronous peak Barrovian metamorphism driven by syn-orogenic magmatism and fluid flow in southern Connecticut, USA

Recent work in Barrovian metamorphic terranes has found that rocks experience peak metamorphic temperatures across several grades at similar times. This result is inconsistent with most geodynamic models of crustal over‐thickening and conductive heating, wherein rocks which reach different metamorphic grades generally reach peak temperatures at different times. Instead, the presence of additional sources of heat and/or focusing mechanisms for heat transport, such as magmatic intrusions and/or advection by metamorphic fluids, may have contributed to the contemporaneous development of several different metamorphic zones. Here, we test the hypothesis of temporally focussed heating for the Wepawaug Schist, a Barrovian terrane in Connecticut, USA, using Sm–Nd ages of prograde garnet growth and U–Pb zircon crystallization ages of associated igneous rocks. Peak temperature in the biotite–garnet zone was dated (via Sm–Nd on garnet) at 378.9 ± 1.6 Ma (2σ), whereas peak temperature in the highest grade staurolite–kyanite zone was dated (via Sm–Nd on garnet rims) at 379.9 ± 6.8 Ma (2σ). These garnet ages suggest that peak metamorphism was pene‐contemporaneous (within error) across these metamorphic grades. Ion microprobe U–Pb ages for zircon from igneous rocks hosted by the metapelites also indicate a period of syn‐metamorphic peak igneous activity at 380.6 ± 4.7 Ma (2σ), indistinguishable from the peak ages recorded by garnet. A 388.6 ± 2.1 Ma (2σ) garnet core age from the staurolite–kyanite zone indicates an earlier episode of growth (coincident with ages from texturally early zircon and a previously published monazite age) along the prograde regional metamorphic T–t path. The timing of peak metamorphism and igneous activity, as well as the occurrence of extensive syn‐metamorphic quartz vein systems and pegmatites, best supports the hypothesis that advective heating driven by magmas and fluids focussed major mineral growth into two distinct episodes: the first at c. 389 Ma, and the second, corresponding to the regionally synchronous peak metamorphism, at c. 380 Ma.     

Metamorphic growth and recrystallization of zircons in extremely O-depleted rocks during eclogite-facies metamorphism: Evidence from U-Pb ages, trace elements, and O-Hf isotopes

A combined in situ SIMS and LA-(MC)-ICPMS study of U-Pb ages, trace elements, O and Lu-Hf isotopes was conducted for zircon from eclogite-facies metamorphic rocks in the Sulu orogen. The two microbeam techniques sampled various depths of zircon domains, revealing different element and isotope relationships between residual magmatic cores and new metamorphic rims and thus the geochemical architecture of metamorphic zircons which otherwise cannot be recognized by the single microbeam technique. This enables discrimination of metamorphic growth from different subtypes of metamorphic recrystallization. Magmatic cores with U-Pb ages of 769 ± 9 Ma have positive δ¹⁸O values of 0.1-10.1‰, high Th/U and ¹⁷⁶Lu/¹⁷⁷Hf ratios, high REE contents, and steep MREE-HREE patterns with negative Eu anomalies. They are interpreted as crystallizing from positive δ¹⁸O magmas during protolith emplacement. In contrast, newly grown domains have concordant U-Pb ages of 204 ± 4 to 252 ± 7 Ma and show negative δ¹⁸O values of −10.0‰ to −2.2‰, low Th/U and ¹⁷⁶Lu/¹⁷⁷Hf ratios, low REE contents, and flat HREE patterns with weak to no Eu anomalies. They are interpreted as growing from negative δ¹⁸O fluids that were produced by metamorphic dehydration of high-T glacial-hydrothermally altered rocks during continental subduction-zone metamorphism. Differences in δ¹⁸O between different domains within single grains vary from 0.8‰ to 12.5‰, suggesting different degrees of O isotope exchange between the positive δ¹⁸O magmatic core and the negative δ¹⁸O metamorphic fluid during the metamorphism. The magmatic zircons underwent three subtypes of metamorphic recrystallization, depending on their accessibility to negative δ¹⁸O fluids. The zircons recrystallized in solid-state maintained positive δ¹⁸O values, and REE and Lu-Hf isotopes of protolith zircon, but their U-Pb ages are lowered. The zircons recrystallized through dissolution exhibit negative δ¹⁸O values similar to the metamorphic growths, almost completely reset U-Pb ages, and partially reset REE systems. The zircons recrystallized through replacement show variably negative δ¹⁸O values, and partially reset REE, and U-Pb and Lu-Hf isotopic systems. Therefore, this study places robust constraints on the origin of metamorphic zircons in eclogite-facies rocks and provides a methodological framework for linking the different types of metamorphic zircons to petrological processes during continental collision.     

华北克拉通古元古代晚期地壳演化和荆山群形成时代制约——胶东地区变质中-基性侵入岩锆石SHRIMP U-Pb定年

胶东地区的荆山群呈近东西向环绕太古宙TTG花岗质片麻岩展布,主要由成熟度高的含石墨变泥砂质岩石、钙硅酸岩和大理岩组成,变质程度达高角闪岩相-麻粒岩相,具孔兹岩系性质。变质中-基性岩侵入到荆山群。它们的侵位时代对于探讨华北克拉通东部元古宙构造演化以及对荆山群沉积时代的制约,都有重要意义。锆石SHRIMP U-Pb定年结果表明,遭受低级变质的闪长岩(S0835)岩浆锆石年龄为1852±9Ma (MSWD=2.1),遭受中高级变质的辉长岩(S0816)变质锆石年龄为1865±11Ma (MSWD=0.76)。结合区域资料,可得出如下结论:(1)荆山群孔兹岩系形成于古元古代晚期(2.2~1.9Ga);(2)古元古代期间,胶东地区从挤压体制转入伸展体制的时间在1.87Ga之前。在华北克拉通中西部的恒山、大青山地区,存在1.97~1.92Ga辉长岩,在1.92~1.83Ga期间发生变质,与本文研究结果类似。这表明华北克拉通中-西部和东部具有类似的古元古代演化历史。     

Zircon growth during subduction and exhumation metamorphism of mafic rocks,Aktyuz terrain,North Tiahshan,Kyrgyzstan

The interpretation of whether a dated metamorphic zircon generation grew during the prograde, peak or retrograde stage of a metamorphic cycle is critical to geological interpretation. This study documents a case at Aktyuz metamorphic terrain, in the southern of Kokchetav‐North Tianshan belt, involving progressive metamorphic recrystallization of mafic rock to eclogite and associated behavior of zircon. Zircons in eclogites are mainly fine grains (5 to 20 μm), and preferentially concentrated with rutile/ilmenite. They also occur as individual grains or clusters in amphibole coronas of garnet. A few larger grains commonly preserve inherited cores and evidence of dissolution and metamorphic outgrowths. Zircon grains separated from amphibolites show inherited zircons with typically magmatic feature, although this become progressively blurred in response to resorption and recrystallization. Mineral inclusions represent epidote‐amphibolite facies in the prograde metamorphism, and the embayed boundary between recrystallized domains and inherited zircons suggest fluid/melt participation. The metamorphic domains are mainly simple overgrowth around the inherited cores or recrystallization domains. The absence of peak metamorphic mineral inclusions and steep pattern of MREE‐HREE indicate no sufficient garnet formed before the metamorphic zircon overgrowth. A tiny rim with homogeneously bright CL image can be distinguished in most zircons. Amphibole inclusions have similar compositions to those in the coronas of garnets, suggesting a retrograde metamorphic origin. The inherited zircon crystallized at 880‐730 Ma, revealing similar age range to the gneiss in Aktyuz area, whereas metamorphic zircon dates prograde metamorphism at 497.9 ±1.4 Ma. In this case, the bulk Zr budget in rocks will become locked into Zr‐bearing minerals during the mafic magma intrusion, when the inherited zircon melting and resorption. The texture shows that metamorphic zircon grew both in the prograde and retrograde stage, and Zr‐bearing magmatic minerals and rutile/ilmenite are by far the main source of Zr for the two stages, respectively.     

Geological framework and Paleozoic tectonic history of the Chinese Altai, NW China: a review

The Chinese Altai, as a key portion of the Central Asian Orogenic Belt (CAOB), is dominated by variably deformed and metamorphosed sedimentary rocks, volcanic rocks and granitic intrusions. Its Early Paleozoic tectonic setting has been variously considered as a passive continental margin, a subduction-accretion complex, or a Precambrian microcontinent, and two representative competing tectonic models have been proposed, i.e., open-closure versus subduction-accretion. Recent studies demonstrate that the high-grade metamorphic rocks previously considered as fragments of a Precambrian basement have zircon U-Pb ages (predominantly 528 to 466 Ma) similar to those of the widely distributed low-grade metasedimentary rocks named as Habahe Group in the region, and all these meta-sedimentary rocks were dominantly deposited in the Early Paleozoic. Petrological evidence and geochemical compositions further suggest that these meta-sedimentary rocks were probably deposited in an active margin, not a passive continental margin as previously proposed. The detrital zircons of sediments and igneous zircons from granitoids including the inherited ones (mainly 543–421 Ma) mostly give positive ?_Hf(t) values, suggesting significant contributions from mantle-derived juvenile materials to the lower crust. A modeling calculation based on zircon Hf isotopic compositions suggests that as much as 84% of the Chinese Altai is possibly made up of “juvenile” Paleozoic materials. Thus, available data do not support the existence of a Precambrian basement, but rather indicate that the Chinese Altai represented a huge subduction-accretion complex in the Paleozoic. Zircon U-Pb dating results for granitoids indicate that magmatism was active continuously from the Early to Middle Paleozoic, and the strongest magmatic activity took place in the Devonian, coeval with a significant change in zircon Hf isotopic composition. These findings, together with the occurrence of chemically distinctive igneous rocks and the high-T metamorphism, can be collectively accounted for by ridge-trench interaction during the accretionary orogenic process.     

U-Pb ages and Hf isotope compositions of detrital zircons from the sandstone in the early cretaceous Wawukuang formation in the Jiaolai Basin,Shandong Province and its tectonic implicationsEI北大核心CSCD

LA-ICP-MS U-Pb dating and in situ Hf isotope analysis were carried out for the detrital zircons to constrain the depositional age and provenance of the Wawukuang Formation, which is believed as the earliest unit of the Laiyang Group in the Jiaolai Basin, and its implications. Most of these detrital zircons from the feldspar quartz sandstone in the Wawukuang Formation are magmatic in origin, which are euhedral-subhedral and display oscillatory zoning in CL images; whereas few Late Triassic detrital zircons are metamorphic in origin and structureless in CL images. U-Pb isotopic dating of 82 zircon grains yields age populations at ca. 129 Ma, 158 Ma, 224 Ma, 253 Ma, 461 Ma, 724 Ma, 1851 Ma and 2456 Ma. U-Pb dating and Hf isotopic results indicate that: 1) the Wawukuang Formation deposited during the Early Cretaceous (129-106 Ma); 2) the detrital zircons with the ages of 1851 Ma and 2456 Ma mainly sourced from the Precambrian basement rocks of the North China Craton; the Neoproterozoic (729-721 Ma) magmatic zircons and the Late Triassic (226-216 Ma) metamorphic zircons sourced from the Su-Lu terrane; The Late Paleozoic detrital zircons could source from the Late Paleozoic igneous rocks in the northern margin of the North China Craton; the Late Triassic (231-223 Ma) magmatic zircons and the 158-129 Ma zircons sourced from the coeval igneous rocks in the Jiaobei and Jiaodong; 3) the deposition age and provenance of the Jiaolai Basin are different from those of the Hefei Basin; 4) the recognition of clastic sediments from the Su-Lu terrane in the Wawukuang Formation suggests that the Su-Lu terrane was under denudation in the Early Cretaceous. ©, 2015, Science Press. All right reserved.     

SHRIMP zircon U–Pb ages of eclogite and orthogneiss from Sulu ultrahigh-pressure zone in Yangkou area,eastern China

We report SHRIMP U–Pb age of zircons in four samples of eclogite and one sample of orthogneiss from Sulu ultrahigh-pressure (UHP) zone in Yangkou area, eastern China. UHP rocks are distributed along the Sulu orogenic belt suturing North China Block with South China Block. In Yangkou area, UHP unit is well exposed for about 200 m along Yangkou beach section and consists mainly of blocks or lenses of ultramafic rocks and eclogite together with para- and orthogneiss which are highly sheared partly. Zircon grains examined in this study from eclogite show oscillatory zoning and overgrowth texture in CL images, and most of the grains have high Th/U ratio ranging from 0.8 to 2.1 indicating an igneous origin. The weighted mean ²⁰⁶Pb/²³⁸U ages of zircons from the four samples range from 690 to 734 Ma. These ages can be correlated to the magmatic stage of the protoliths. In rare cases, zircon grains possess a narrow rim with very low Th/U ratio (< 0.02). EPMA U–Th-total Pb dating of such rim yields younger ages that range from 240 to 405 Ma marking the metamorphic stage. On the other hand, zircons from the orthogneiss show irregular shape and zoning with inclusion-rich core and inclusion-free rim. These grains of zircon yield U–Pb discordia intercept ages of 226 ± 63 Ma and 714 ± 110 Ma (MSWD 0.78). Bulk of the areas of the rims rim of the zircons demonstrate younger ²⁰⁶Pb/²³⁸U ages close to the upper intercept, with low Th/U ratio (< 0.20) indicating their metamorphic origin. In contrast, the cores show older ²⁰⁶Pb/²³⁸U ages close to lower intercept and high Th/U ratio of (0.14–5.25) indicating their igneous origin. The upper intercept age is also commonly noted in zircons from eclogite. Our results suggest a bimodal igneous activity along this zone during the Neoproterozoic, probably related to the rifting of the Rodinia supercontinent.     

CL-imaging and ion microprobe dating of single zircons from a high-grade rock from the Central Zone,Limpopo Belt,South Africa: Evidence for a single metamorphic event at ∼2.0 Ga

The combination of ion microprobe dating and cathodoluminescence (CL) imaging of zircons from a high-grade rock from the Central Zone of the Limpopo Belt were used to constrain the age of metamorphic events in the area. Zircon grains extracted from an orthopyroxene-gedrite-bearing granulite were prepared for single crystal CL-imaging and ion microprobe dating. The grains display complex zoning when using SEM-based CL-imaging. A common feature in most grains is the presence of a distinct core with a broken oscillatory zoned structure, which clearly appears to be the remnant of an original grain of igneous origin. This core is overgrown by an unzoned thin rim measuring about 10–30 μm in diameter, which is considered as new zircon growth during a single metamorphic event. Selected domains of the zircon grains were analysed for U, Pb and Th isotopic composition using a CAMECA IMS 1270 ion microprobe (Nordsim facility). Most of the grains define a near-concordant cluster with some evidence of Pb loss. The most concordant ages of the cores yielded a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2689 ± 15 (2σ) Ma, interpreted as the age of the protolith of an igneous origin. The unzoned overgrowths of the zircon grains yielded a considerably younger weighted mean ²⁰⁷Pb/²⁰⁶Pb age of ∼2006.5 ± 8.0 Ma (2σ), and these data are interpreted to reflect closely the age of the ubiquitous high-grade metamorphic event in the Central Zone. This study shows clearly, based on both the internal structure of the zircons and the data obtained by ion microprobe dating, that only a single metamorphic event is recorded by the studied 2.69 Ga old rocks, and we found no evidence of an earlier metamorphic event at ∼2.5 Ga as postulated earlier by some workers.     

Mesoarchean mafic dykes of the Belomorian eclogite province (Gridino Village Area,Russia)

Archean processes of eclogitization in the Gridino metamorphic association (the Belomorian eclogite province) developed in mafic dykes, boudins, and acidic rocks of the Archean continental crusts. To determine the U-Pb age of the intrusion of the latest dykes, the geochronological samples were taken from the dyke of ferriferious metagabbro that cross-cuts the dyke of eclogitzed and granulitized olivine gabbronorite. The igneous zircons were dated by the SHRIMP II technique. The zircons showed a concordia age of 2846 ± 7 Ma, which is considered as the time of intrusion of a mafic melt. The younger low-thorium zircon rims of 2.78–2.81 Ga age around the igneous cores are typical formations that appeared under metamorphic conditions in equilibrium with a migmatite melt, and may characterize the time of formation of the granite leucosome under metamorphism, probably of eclogite facies.     

Zircon trace element characteristics and ages in granulite xenoliths: a key to understanding the age and origin of the lower crust,Arkhangelsk kimberlite province,Russia

Garnet granulite and pyroxenite xenoliths from the Grib kimberlite pipe (Arkhangelsk, NW Russia) represent the lower crust beneath Russian platform in close vicinity to the cratonic region of the north-eastern Baltic (Fennoscandian) Shield. Many of the xenoliths have experienced strong interaction with the kimberlite host, but in others some primary granulite-facies minerals are preserved. Calculated bulk compositions for the granulites suggest that their protoliths were basic to intermediate igneous rocks; pyroxenites were ultrabasic to basic cumulates. A few samples are probably metasedimentary in origin. Zircons are abundant in the xenoliths; they exhibit complex zoning in cathodoluminescence with relic cores and various metamorphic rims. Cores include oscillatory zircon crystallized in magmatic protoliths, and metamorphic and magmatic sector-zoned zircons. Recrystallization of older zircons led to the formation of bright homogeneous rims. In some samples, homogeneous shells are surrounded by darker convoluted overgrowths that were formed by subsolidus growth when a change in mineral association occurred. The source of Zr was a phase consumed during a reaction, which produced garnet. Late-generation zircons in all xenoliths show concordant U–Pb ages of 1.81–1.84 Ga (1,826 ± 11 Ma), interpreted as the age of last granulite-facies metamorphism. This event completely resets most zircon cores. An earlier metamorphic event at 1.96–1.94 Ga is recorded by some rare cores, and a few magmatic oscillatory zircons have retained a Neoarchaean age of 2,719 ± 14 Ma. The assemblage of metaigneous and metasedimentary rocks was probably formed before the event at 1.96 Ga. Inherited magmatic zircons indicate the existence of continental crust by the time of intrusion of magmatic protoliths in the Late Archaean. The U–Pb zircon ages correspond to major events recorded in upper crustal rocks of the region: collisional metamorphism and magmatism 2.7 Ga ago and reworking of Archaean rocks at around 1.95–1.75 Ga. However, formation of the granulitic paragenesis in lower crustal rocks occurred significantly later than the last granulite-facies event seen in the upper crust and correlates instead with retrograde metamorphism and small-volume magmatism in the upper crust.     

Granulite facies metamorphism, palaeo-isotherms and disturbance of the U-Pb systematics of zircon in anorogenic plutonic rocks from the Adirondack Highlands

Abstract In the Adirondack Highlands of New York State, the effect of granulite facies metamorphism on the physical and isotopic characteristics of zircon from anorogenic plutonic rocks has a distinct geographical pattern. The location of zircon populations which appear to have been altered describes a roughly circular area where metamorphic palaeotemperatures have been determined to be in excess of 750° C. Zircons from anorogenic plutonic rocks outside this area were undisturbed during metamorphism and yield well constrained ages. Granitic, charnockitic and mangeritic anorogenic plutonic rocks peripheral to the Marcy anorthosite massif have large, euhedral, prismatic zircons that display fine, internal, magmatic growth zonations and abundant, randomly orientated, mineral inclusions. Co-genetic zircon fractions yield linear discordant arrays and well constrained upper intercepts of 1125–1157 Ma. Metamorphic zircon is limited to sporadically developed and volumetrically insignificant, clear, low-U overgrowths or protuberances. In marked contrast, zircons from petrographically and geochemically identical rocks adjacent to, or within, the Marcy anorthosite massif are typically large, limpid, anhedral to subhedral crystals or crystal fragments lacking internal features except for tubular cavities and CO-₂-rich inclusions. Co-genetic zircon fractions yield nearly concordant, non-linear clusters with ²⁰⁷Pb/²⁰⁶Pb minimum ages of 1073–1095 Ma. Metamorphic overgrowths cannot be readily identified by optical or cathodoluminscence techniques; however, many grains show complex and unusual external boundaries suggestive of post-crystallization modification. These data indicate that temperatures as low as 750° C, in combination with other factors, may have been sufficient to facilitate recrystallization, and diffusion of radiogenic Pb from the zircon crystal structure, during the complex, protracted metamorphism of the Adirondack Highlands.     

Does ternary feldspar constrain the metamorphic conditions of high‐grade meta‐igneous rocks? Evidence from orthopyroxene granulites,Bohemian Massif

The presence of ternary feldspar in high‐grade meta‐igneous rocks, and the recognition of the thermometric significance of this mineral, has led recent researchers to postulate peak metamorphic temperatures in excess of 1000 °C. However, it needs to be established that such ternary feldspar is not in fact a survivor of the original high‐temperature crystallization of the igneous protolith. After exsolution, the host and lamellae in the ternary feldspar grains may be stable throughout subsequent history as long as recrystallization does not occur. Such a history may involve rehydration and metamorphism, including H₂O‐saturated conditions, with the compositions and proportions of the host and lamellae being modified to reflect the P–T conditions experienced. In the case of the high‐grade meta‐igneous rocks from the Moldanubian of the Bohemian Massif, some samples that contain ternary feldspar preserve a substantial measure of their igneous heritage. Orthopyroxene‐bearing granulites not only include types that are barely affected by the metamorphism, but also others that have undergone hydration of the igneous protolith prior to the development of a metamorphic overprint. A key to establishing the igneous origin of the ternary feldspar grains is their preservation in garnet that is either itself igneous, or of a relatively low‐temperature metamorphic origin. Applying the logic to the other ternary feldspar‐bearing meta‐igneous rocks deprives the Moldanubian of its ultrahigh temperature (UHT) metamorphic status.